The present invention generally pertains to the field of metal matrix composites (MMCs). More particularly, the present invention is related to machining of MMCs.
In recent years there have been numerous improvements in the equipment and in the processing technology for the manufacturing of metal matrix composites (MMCs). The most commercially advanced of these MMCs is the family of stir-cast composites where either low cost silicon carbide or aluminum oxide particulate is mixed into molten aluminum and subsequently solidified in the form of an ingot or a billet. Almost any aluminum alloy can be used as a matrix and the level of particulate can be readily varied from less than 10 to over 30 volume percent. Those MMCs cast into billet or ingot form can be further hot worked through extrusion, rolling, and forging; and foundry MMCs can be remelted and cast into various shapes using modified aluminum foundry procedures. These MMCs, in all of their forms, exhibit the elevated modulus, strength, wear resistance, lowered coefficient of thermal expansion, and many other useful parameters while having a density only slightly above that of aluminum.
The industrialization of stir-cast MMCs has made them available in large quantities and at a low price. The present average price of these MMCs is less than $2.00/lb and a price closer to $1.00/lb is anticipated for the future. These materials are viewed as replacements for existing metals and resin matrix composites where higher performance at low cost is desired. Extruded stir cast billets are currently being used as the drive shaft in some sports cars, certain police vehicles, and various light trucks. Cast foundry MMCs are also being used as the rear brake rotors for the various top-end vehicles, and the MMCs are utilized as brakes in several electric vehicles. Other large volume automotive applications for MMCs under consideration are pump and differential housings, brake calipers, and pulleys.
The use of MMCs in high production applications necessitates low fabricating and finishing costs. Applications such as automotive disk brakes require extensive final machining which must be performed rapidly. To take advantage of the low MMC material costs and improved properties, MMC components must be machined efficiently at a cost near that of common engineering metals. However, since the early efforts in MMC development, many thousands of MMC prototypes and limited production components have been machined using various conventional and non-conventional techniques. There have even been a number of machining studies carried out on simple operations such as drilling, milling, and lathe work which have implied that machining of MMCs is well in hand. In spite of this body of empirical data, the large automotive companies maintain that one of the major barriers to the use of MMCs for brake rotors is the high cost of final machining. As a result, it is a commonly held belief that machining of metal matrix composites is too costly for most commercial high-production applications.
The unusual and sometimes difficult machining characteristics of MMCs are a result of the two phase microstructure consisting of the relatively soft and ductile continuous aluminum alloy matrix reinforced by hard ceramic particles. The average particle size may vary from 8-25 microns with certain composites reinforced with particles of 45 microns or larger. The size of reinforcement has a pronounced affect on machinability and cutting tool life. Many metals have two phase structures, such as cast iron or high silicon aluminum alloys, but in no metal is the difference in hardness as pronounced as in the metal matrix composite. The soft continuous aluminum matrix necessitates that the cutting tool have a sharp edge and have the ability to cut through the matrix while producing discrete chips of material. Since the aluminum matrix has low hardness and modest strength, the theoretical energy required for machining (on a macroscopic scale) is low, and high metal removal rates should be possible.
Unfortunately, the difficulty in machining MMCs arises from the tendency of the hard and abrasive particles thereof to dull the cutting edge of a conventional tool. Conventional high speed steel tools, regardless of the tungsten or cobalt content, do not last more than a few revolutions in contact with the MMC before the cutting edge is dulled. In one conventional approach a tool of sintered tungsten carbide is used in an attempt to machine the MMC. Sintered tungsten carbide is much harder and lasts longer before the edge is dulled by a combination of abrasion and chipping due to the impact of the moving ceramic particles in a rotating work piece on the sharp, but brittle, sintered tungsten carbide tool edge. MMCs containing large particles generate higher kinetic energy and are more likely to cause tool edge chipping. This type of localized cutting edge fracture may be more detrimental to the cutting edge than classical wear. Although they do not demonstrate long tool life, tungsten carbide cutting tools are currently used, with limited success and much difficulty, for certain MMC machining operations and prototype fabrication. MMC finishing by grinding with abrasive grain wheels has never been completely successful as the soft aluminum matrix tends to quickly load the wheels making consistent metal removal impractical.
In another attempt to machine MMCs, various specialized machining techniques such as EDM (electro-discharge machining), laser cutting, and abrasive water jet cutting have been employed. While these techniques have some limited utility, these processes tend to be specialized and very expensive. Hence, these techniques, like the use of sintered tungsten carbide tools, are not applicable for three-dimensional, high production machining of MMCs.
Another conventional approach for machining particle reinforced aluminum matrix composites is to use diamond superabrasives, specifically, single point polycrystalline diamond (PCD) cutting tools. These synthetic polycrystalline diamonds are produced in the form of thin sheets, approximately 0.040 inch (1 mm thick), and have varying grain or crystallite size. The sheets are laser cut into small segments, sintered to tungsten carbide substrates, and then diamond honed to produce a sharp cutting edge on drills, end mills, turning tools, and many other standard insert shapes. PCD tools are flat and do not incorporate any of the complex tool geometries and chip breaking features so common to high speed steel or tungsten carbide inserts and, as a result, have generally been limited to simple machining operations. CVD (chemical vapor deposition) diamond coating technology has been developed to permit the applications of very thin diamond coatings to complex carbide tool shapes. When used for external and internal turning, some milling, and drilling operations on MMCs containing modest loading of particulate, the PCD tools can have reasonable tool life and produce good surface finish.
Unfortunately, the use of PCD tools to machine MMCs has some significant disadvantages. As an example, PCD tools are expensive, costing between 5 and 20 times that of sintered tungsten carbide tools. Whereas a typical tungsten carbide insert will have at least 3 (and as many as 6) cutting points which can be used before the insert is discarded, the PCD insert will only have one. As yet another example of the disadvantages associated with PCD tools, PCD inserts can only take light interrupted cuts and the depth of continuous cuts is limited by the size of the PCD chip in the insert. A further disadvantage to PCD tools lies in the fact that diamond tipped or veined drill bits and end mills are extremely sensitive to impact and uneven loading. Even a slight deviation in the alignment during drilling a hole can irreparably damage a small tool costing many hundreds of dollars. Additionally, PCD tools are, disadvantageously, fragile and require special care to prevent catastrophic failure. As still another significant disadvantage, fluid cooling must be carefully controlled during the use of PCD tools to prevent fracture by thermal shocking of the synthetic polycrystalline diamond.
Further significant drawbacks associated with PCD tools pertain to the fact that, when used to machine MMCs, PCD tools do wear and the edge thereof becomes dull. In a manner similar to tungsten carbide, PCD wear is at least in part a function of chipping damage caused by the shock of ceramic particles in the spinning MMC work piece striking the sharp edge of the synthetic polycrystalline diamond. This condition is most severe in composites where the mean particle size is large and the kinetic energy is high. Although larger PCD grain size may offer increased resistance to chipping as does a greater tool tip radius, even single crystal natural diamond cutting tools become dull by chipping of the cutting edge and do not offer appreciably longer tool life. Thin diamond coatings generally have very poor tool life as the coating is rapidly fractured and stripped away by contact with the particulate.
An additional problem associated with the use of single point PCD tools to machine MMCs is the difficulty in making shallow cuts of approximately 0.001 of an inch (0.025 mm). If the depth of cut is much less than 0.010 of an inch, the MMC cannot be effectively cut by the tool. It is believed that this shortcoming of PCD tools is because the PCD tool edge quickly becomes dull on a microscopic scale. If the depth of cut is small, the MMC will deform under the pressure at the tool tip rather than allowing the tool to penetrate and establish conventional cutting action.
As still another drawback, to maximize tool life, PCD tools and inserts must be removed from service and rehoned at specific intervals. This procedure is not consistent with high speed production machining where tungsten carbide inserts are used and discarded at precise intervals. As a result of the limitations of PCDs many machine shops believe that these tools are not practical nor economical for the rapid material removal required for high volume machining of MMCs.
Recently, a new method of machining hardened ferrous and nickel based monolithic metals has been introduced. This technique is called high efficiency deep grinding (HEDG). In this process, tools are made by plating thousands of 60-80 mesh boron nitride superabrasive particles to a hardened steel disk or symmetrical shaped form. The density of particulate usually is in excess of 1000 particles/square inch. These tools are rotated at high speeds (over 15000 fps) on rigid spindles. High contact pressures are required to produce the desired metal removal rates. Using various cutting lubricants ranging from water soluble oil to hydrocarbon cutting fluid, these tools are used to machine hardened steel at feed rates equal to those employed in the machining of low carbon steel. The size and shape of the boron nitride particles, as well as the particle loading on the disk substrate, are adjusted to maximize cutting efficiency. Although the process resembles grinding or abrasive disk cutting of ceramics, the method of metal removal is more closely related to classical milling. The contact between the many boron nitride particles and the work piece produces microscopic fine chips, not the powdery swarf commonly associated with grinding processes. This machining characteristic may be one of the reasons that high metal removal rates are possible. Since there are many particles which act as miniature cutting edges the life of the tools has been high even when machining hardened tool steel. The blocky but sharp edged boron nitride particles are not susceptible to fracture and tool wear is low. The low cost of boron nitride particulate enable the cutting wheels to be manufactured inexpensively. Worn wheels may be reused by replating with new sharp particles. However, boron nitride is not the optimum material for cutting non-ferrous based MMCs. While the relatively high density of BN particles on the surface of these cutting tools works adequately with hardened steel where chips are very hard and small, a similar tool is not compatible with MMCs where the loading with soft aluminum swarf would quickly prevent further cutting action. Similarly, the small 60-80 mesh particles used in HEDG machining of steels would also encourage rapid loading with MMC swarf.
Thus, a need exists for a method and apparatus which provides for machining of metal matrix composites (MMCs) having hard and abrasive particulate together with a soft metal matrix. A further need exists for a method and apparatus which meets the above need and which does not suffer from the rapid dulling associated with conventional machining tools. Still another need exists for a method and apparatus which meets the above needs and which does not suffer from the expense and fragility associated with conventional machining approaches. Yet another need exists for a method and apparatus which meets the above needs and is more suitable for rapid MMC machining and which enables deep, heavy, interrupted cuts without the risk of loss of a sole cutting edge.
The present invention provides a method and apparatus which provides for machining of metal matrix composites (MMCs) having hard and abrasive particulate together with a soft metal matrix. The present invention further provides a method and apparatus which achieves the above accomplishments and which does not suffer from the rapid dulling associated with conventional machining tools. The present invention further provides a method and apparatus which achieves the above accomplishments and which does not suffer from the expense and fragility associated with conventional machining approaches. The present invention also provides a method and apparatus which achieves the above accomplishments and is more suitable for rapid MMC machining and which enables deep, heavy, interrupted cuts without the risk of loss of a sole cutting edge.
In one embodiment, the MMC machining tool of the present invention is comprised of a support disk. In this embodiment, the MMC machining tool is further comprised of a plating material which is bonded to at least a portion of the support disk. The present embodiment further recites a plurality of diamond particles coupled to the plating material such that the plurality of diamond particles are securely plated to at least a portion of the support disk. In this embodiment, the plurality of diamond particles are securely plated to the support disk in a manner such that the plurality of diamond particles are adapted to machine metal matrix composite material.
In another embodiment, the MMC machining tool of the present invention includes the features of the above described embodiments and further recites that the plurality of diamond particles are securely brazed to at least a portion of the support disk.